The invention relates generally to a Local Area Network (LAN) wherein each hub is connected to a plurality of workstations by means of links having various lengths and relates in particular to a system for automatically adapting the data rate exchanged between a hub and a workstation in a LAN.
In a Local Area Network (LAN) such as the ETHERNET network, each hub can be connected to a plurality of workstations by means of links having various lengths. But, the problem is that the transmission rate on a link can depend upon the length of the link. As a matter of fact, the minimal length of a frame in such a network depends on the go and return period on the link which corresponds to the time during which it is possible to detect a possible collision.
This requirement corresponds to the following inequation:
L/Cxe2x89xa72l/V
wherein:
V is the signal propagation speed on the link
C is the capacity of the network in bits/s
L is the minimal length of a frame in bits
1 is the length of the link.
This means that for a given rate at which the data are transmitted over the network corresponding to a given capacity C, there is a maximal length for the link connecting the hub to the workstation. Accordingly, if the system is configured in such a way that a new workstation would require to be connected at a length greater that the maximum allowed by the above inequation, the only way to solve the problem is to slow down the data rate. Thus, most of the fast hubs of an ETHERNET network have the capability to switch from 100 Mbits/s to 10 Mbits/s. However, assuming that a rate of 100 Mbits/s corresponds to a maximum length of 100 m, a rate of 50 Mbits/s should be allowed on a link of a 200 m length. Switching to a data rate of 10 Mbits/s means that the data are exchanged at a rate which is 5 times inferior to the rate which could be normally used and therefore makes the transmission more expensive for the customer.
Accordingly, the object of the invention is to provide a system and a method for automatically adapting the data rate over a link connecting a hub to a workstation in a Local Area Network (LAN) in function of the length of the link.
Therefore, the invention relates to a method for adapting the frequency of the transmission rate on a link connected between a hub including a hub adapter and at least a workstation including a workstation adapter, said rate being based on a frequency which is inversely proportional to the length of the link. This method comprises the steps of sending from the hub adapter to the workstation adapter a check frame at a rate based on a frequency value VCLK having a value comprised between a minimum frequency F1 and a maximum frequency F2, and sending from the workstation adapter to the hub adapter an acknowledge frame at a rate based on frequency F1 only if the workstation adapter has received the check frame within a predetermined interval of time.
According to a second aspect, the invention relates to an automatic speed adaptation system in a Local Area Network (LAN) between a hub including a hub adapter and at least a workstation including a workstation adapter for exchanging data over a link connected between the hub adapter and the workstation adapter at a rate based on a frequency which is inversely proportional to the lengths of the link. Each of said hub adapter and workstation adapter comprises a clock generator for generating a clock having a frequency between F1 and F2 and processing means for transmitting at least a check frame from the hub adapter to the workstation adapter at a rate based on a frequency VCLK generated by the clock generator under the control of the processing means and selected as being the frequency corresponding to the length of the link, and for transmitting an acknowledge frame from the workstation adapter to the hub adapter thereby ascertaining that the selected frequency is the right frequency resulting in the best quality of transmission.